Site-Specific Information 

in Support of 

Establishing Numeric Nutrient 

Criteria in the Halifax River Estuary

Florida Department of Environmental Protection

Tallahassee, FL 32399

August 23, 2010 
Draft ii August 2010 

Table of Contents

Executive Summary............................................................................................................................1

Geographic and Physical Description ..................................................................................................4

Sources of Nutrients...........................................................................................................................6

Biological Summary ...........................................................................................................................7

Oyster Reefs.............................................................................................................................................. 7

Salt Marsh................................................................................................................................................. 7

Plankton .................................................................................................................................................... 8

Water Quality Data Sources and Studies.............................................................................................8

Temporal Considerations and the Reference Period Method ................................................................. 12

Waters on the 303(d) List.................................................................................................................14

Numeric Nutrient Criteria Recommendation.....................................................................................20

Reference Period .....................................................................................................................................20

Chlorophyll a/[TN] or [TP] Regression Models........................................................................................20

General Models.......................................................................................................................................21

Results and Discussion .....................................................................................................................21

Spatial Considerations and Segmentation..............................................................................................21

General Models.......................................................................................................................................28

Proposed Numeric Nutrient Targets ................................................................................................. 31

Methodology...........................................................................................................................................31

Magnitude...............................................................................................................................................32

Summary and Conclusions................................................................................................................33

References.......................................................................................................................................35
Draft iii August 2010 

List of Tables

Table 1a. Summary of lines-of-evidence results: loading and concentration limits for the north 

HRE.............................................................................................................................................. 1

Table 1b. Summary of lines-of-evidence results: loading and concentration limits for the south 

HRE.............................................................................................................................................. 2

Table 2. Checklist of nutrient enrichment symptoms in the HRE. ................................................................ 3

Table 3. Land uses in the HRE and watershed (SJRWMD 2004)................................................................... 4

Table 4a. Estimates of current (circa 2004) external loadings of TN and TP to the north HRE. .................. 6

Table 4b. Estimates of current (circa 2004) external loadings of TN and TP to the south HRE. .................. 7

Table 5. Waterbodies on the 2010 Verified List for impairments in the Halifax River Planning Unit

(FDEP). ......................................................................................................................................14

Table 6. Estuarine waterbodies in the Halifax River Planning Unit that are proposed for delisting 14

Table 7a. HRE reference period results—first line of evidence: Annual median, 2000–08.......................25

Table 7b. HRE reference period results—first line of evidence: Maximum wet-season median, 

2000–08 (July–September) ......................................................................................................26

Table 8a. HRE linear regression results–second line of evidence: Annual median (1992–2008; 

annual average data) ...............................................................................................................27

Table 8b. HRE linear regression results–second line of evidence: Maximum wet-season median 

(1992–2008; average monthly; July–September only for TN; all months for TP).....................27

Table 9a. Estimates of current (circa 2004) external loadings of TN and TP to the north HRE. ................28

Table 9b. Estimates of current (circa 2004) external loadings of TN and TP to the south HRE. ................29

Table 10a. North HRE current loading (loading limits) for TN and TP with the model-predicted 

concentrations. .........................................................................................................................31

Table 10b. South HRE current loading (loading limits) for TN and TP with the model-predicted 

concentrations. .........................................................................................................................31

Table 11a. Summary of lines-of-evidence results: Loading and concentration limits for the north 

HRE............................................................................................................................................34

Table 11b. Summary of lines-of-evidence results: Loading and concentration limits for the south 

HRE............................................................................................................................................34

List of Figures

Figure 1. Halifax River Planning Unit, Including the 1998 303(d) list and Planning List waters. ................. 5

Figure 2. Spatial distribution of salinity and turbidity (mean and standard deviation) by sampling 

site. ............................................................................................................................................. 9

Figure 3. Spatial distribution of TSS (mean and standard deviation) by sampling site..............................10

Figure 4. Spatial distribution of chlorophyll a, TN, and TP (mean and standard deviation) by 

sampling site.............................................................................................................................11

Figure 5. Spatial distribution of TSI values (means) for two periods, 1992–2000 and 2000–08. ..............12

Figure 6. Temporal trends in TN, TP, and chlorophyll a for the HRE overall, 1992–2008..........................13

Figure 7. Percent composition of major land uses in the south and north HRE (based on 2004 

imagery)....................................................................................................................................22
Draft iv August 2010 

Figure 8a. Location of tidal node at boundary between the north and south HRE....................................22

Figure 8b. Location of tidal node at boundary between the HRE and Matanzas River Estuary near 

High Bridge. .............................................................................................................................. 23

Figure 9. Distribution of oyster and other significant habitat communities in the HRE. ...........................24

Figure 10. VCEMD water quality sampling sites. .......................................................................................25

Figure 11. Average monthly TN, chlorophyll a, and TP in the HRE overall. Monthly rainfall data 

are shown for comparison. The 1992–2008 period was used for both water quality 

and rainfall data. ......................................................................................................................26

Figure 12. Mean turbidity by year (1992–2008) for the north and south HRE compared with the 

IRL. ............................................................................................................................................28

Figure 13. The log-linear plot of TN and TP load limits (g/m2

/yr) vs. hydraulic loading (qs). A 

point lying above the 95% confidence interval for the line-of-best-fit (= upper 

mestrophy) is considered eutrophic; below the lower mesotrophic line (LML) is 

considered oligotrophic (based on Steward and Lowe 2010)...................................................30
Draft 1 August 2010 

Executive Summary

This report was prepared by the Florida Department of Environmental Protection (FDEP), in cooperation 

with local scientists, to support the development of numeric nutrient criteria for the Halifax River 

Estuary (HRE). The primary purpose of the proposed numeric nutrient criteria is to protect healthy, 

well-balanced natural populations of flora and fauna from the effects of excess nutrient enrichment. 

The Halifax River is a 40-kilometer-long tidal estuary located on the Atlantic coast near Daytona Beach, 

with its major ocean connection situated at Ponce de Leon Inlet (Figure 1). The basin morphology is 

typical of the Florida east coast, with a series of flat, low-gradient terraces separated by ancient barrier 

beaches (Scholl et al. 1980). The Halifax River drains approximately 963 square kilometers (km2

), of 

which roughly 18% is urbanized (St. Johns River Water Management District [SJRWMD] 2004 Land Use). 

From 1990 to 2006, the population in Volusia County grew approximately 34% (http://www

protectourwaters.org/). 

Nutrients to the HRE originate from stormwater runoff, agriculture, silviculture, urban/suburban land 

uses, atmospheric deposition, sediment loading and turbidity from shoreline erosion, septic tanks, and 

wastewater treatment facilities (WWTFs). 

Oyster beds are the primary sensitive aquatic resource in the HRE, roughly analogous in form and 

function to seagrasses in other estuaries throughout Florida. Unfortunately, there is no functional 

relationship between nutrient levels and oysters (as there is with seagrasses) that would be useful for 

setting nutrient limits in the estuary. In fact, additional nutrient loads may actually increase oyster 

production. Therefore, FDEP has evaluated multiple lines of evidence associated with the trophic 

state/chlorophyll a/nutrient relationship in the HRE to develop nutrient criteria that would maintain 

designated uses. The evidence presented in this document shows that aquatic life designated use in the 

HRE is fully supported. FDEP therefore proposes that the numeric nutrient criteria be crafted to 

maintain the existing nutrient regime. 

Numeric nutrient criteria were developed from data collected at 20 stations by the Volusia County 

Environmental Management Department (VCEMD) from 1992 to 2008. For the data analysis, the HRE

was divided into a northern and a southern segment based on local land uses and hydrodynamic 

properties, particularly water residence time, which greatly influences water quality and trophic 

response (Steward and Lowe 2010). The HRE system appears to be nitrogen (N) limited, with a total 

nitrogen:total phosphorus (TN:TP) ratio of less than 5 (by weight). 

Chlorophyll a concentrations in the HRE tend to correlate better to TN than to TP (Steward and Lowe 

2010). Additional data from the University of Florida suggest N limitation (E. Phlips, pers. comm.). TN 

has generally declined in the HRE since the 1990s, corresponding to reductions in chlorophyll a and 

trophic status (Steward and Lowe 2010). 

Three lines of evidence were used in developing numeric nutrient criteria for the HRE: a reference 

period method, chlorophyll a–nutrient regression analyses, and two estuary empirical models (Steward 

and Lowe 2010; Dettman 2001). Tables 1a and 1b summarize the three lines of evidence and the 

proposed loading and concentration limits for the north and south segments, respectively. 

Table 1a. Summary of lines-of-evidence results: loading and concentration limits for the north HRE.
Draft 2 August 2010 

Target condition is mesotrophy (chlorophyll a targets are an annual median of 4.5 micrograms per liter [μg/L] 

and a seasonal median of 7.9 μg/L for July through September) 

- = Empty cell/no data

kg/yr = Kilograms per year

mg/L = Milligrams per liter

N/A = Not applicable a 0.62 mg/L TN and 0.2 mg/L TP are predicted concentrations based on current loadings (circa 2004). 

- TN 

(kg/yr)

TN 

(kg/yr) 

TP 

(kg/yr)

TP 

(kg/yr) 

Loading Limit (current loading) 257,832 257,832 43,494 43,494

- Median (mg/L)

Annual

Median (mg/L)

July–September

Median (mg/L)

Annual

Median (mg/L)

July–September

Reference Period (2000–08) 0.61 0.90 0.12 0.18

Regressions (1992–2008 data) 0.58 0.90 Not significant 0.17

General Models 0.62 N/A 0.2 N/A

Table 1b. Summary of lines-of-evidence results: loading and concentration limits for the south HRE. 

Target condition is oligo-mesotrophy (chlorophyll a targets are an annual median of 3.5 μg/L and a seasonal 

median of 5.8 μg/L for July through September)

- = Empty cell/no data

N/A = Not applicable a The midpoint between the two values in parentheses = 0.135 mg/L TP. The two [TP] values reported in parentheses are the results from the 

lower mesotrophic and oligotrophic [TP] model equations applied to the south HRE (see the section on General Considerations and Methods). 

- TN 

(kg/yr)

TN 

(kg/yr)

TP 

(kg/yr)

TP 

(kg/yr)

Loading Limit (current loading) 222,000 222,000 38,000 38,000

- Median (mg/L) 

Annual

Median (mg/L)

July–September

Median (mg/L)

Annual

Median (mg/L)

July–September 

Reference Period (2000–08) 0.39 0.51 0.12 0.19

Regressions (1992–08 data) 0.30 0.41 Not significant 0.15

General Models 0.42 N/A 0.135 (0.09, 0.18)a N/A
Draft 3 August 2010 

Table 2. Checklist of nutrient enrichment symptoms in the HRE. - = Empty cell/no data

N/A = Not applicable 

Response Variable Observed 

Historically?

Observed 

Currently?

Explanation Source

Low dissolved oxygen (DO) 

(hypoxia/anoxia) 

Yes Yes There are naturally low DO levels in some of the tidal creeks 

and in Rose Bay.

FDEP 2008 

Reduced clarity Yes Yes Naturally occurring high turbidities with potential exacerbation 

by human activities.

Steward et al. 

2010 

Increased chlorophyll a concentrations Yes No No annual average exceeds the 11 μg/L threshold for estuaries. 

General decreasing trend in chlorophyll a and Trophic State 

Index (TSI) from 1992 to 2008.

FDEP 2008; 

Steward et al. 

2010

Phytoplankton blooms No No - Steward et al. 

2010

Problematic epiphyte growth N/A N/A - Steward et al. 

2010

Problematic macroalgal growth No No - Steward et al. 

2010

Submerged aquatic vegetation (SAV) 

community changes or loss

N/A N/A Naturally high turbidities and high flushing rates have 

prevented SAV establishment.

Steward et al. 

2010

Emergent or shoreline vegetation 

community changes or loss 

Yes Yes Roughly 50% and 75% of the shoreline hardened in south HRE 

and north HRE, respectively. Dredge-and-fill activities primarily 

responsible for the disappearance of shoreline vegetation, not 

nutrients.

Steward et al. 

2010

Oyster reef community changes or loss Yes Yes Roughly 50% and 75% of the shoreline hardened in the south 

and north HRE, respectively. Dredge-and-fill activities primarily 

responsible for the disappearance of oyster reefs, not 

nutrients.

Steward et al. 

2010

Impacts to benthic community Yes No A 1979 assessment of the benthic macroinvertebrate 

community found an extremely depauperate fauna with a total 

of 36 species present and only one mollusk was found in 

appreciable numbers.

Diaz et al. 

1980

Magley 1990

Fish kills No No - Florida Fish 

and Wildlife 

Conservation 

Commission 

(FWCC) 2010

Other N/A N/A - 
Draft 4 August 2010 

Geographic and Physical Description

The HRE is located in the Upper East Coast Drainage Basin (Figure 1) and consists of 53 segments with 

waterbody identification (WBID) numbers (FDEP 2008). It includes portions of Flagler and Volusia Counties and 

encompasses an area of nearly 87,986 hectares (ha) (208,000 acres). Major sources of fresh water to the 

estuary include Bulow Creek, the Tomoka River, and Spruce Creek, and their natural tributaries. Both the 

Tomoka River and Spruce Creek are designated as Outstanding Florida Waters (OFWs). 

The HRE is 37.5 kilometers long and 914 meters wide on average, and has a total area of 35.8 km2 (Steward et 

al. 2010). The average slack tide depth is 1.1 meters (m) (Scholl et al. 1980). Rainfall has averaged 50 inches 

per year over the past 30 years, with a rainy season occurring from June through September (SJRWMD 1998). 

Six causeways cross the HRE and inhibit circulation in the area (SJRWMD 2003). The Intracoastal Waterway 

intersects the Halifax River and is frequently used by large vessels transiting north and south. The watershed 

area (87,986 ha) to estuary area (3,581 ha) ratio is approximately 24 to 1, indicative of a potentially large 

volume of runoff into a comparatively small estuarine system (Steward et al. 2010). Table 3 lists the major 

land uses and their associated percentages in the Halifax watershed, the majority of which (70%) is a 

combination of water, wetlands, rangeland, and forested lands.

Mosquito impoundments, residential development, and silviculture have altered a majority of the estuary’s 

historical watershed drainage. Ormond Beach, Holly Hill, Daytona Beach, and Bethune Point are all relatively 

densely populated residential areas in the watershed. 

Some salient physical characteristics of the HRE are as follows:

Length of system—~23 miles (mi) (37.5 km), wholly within Volusia County;

Average width of estuary—0.56 mi (0.9 km);

Area of estuary—~13.8 square miles (mi2

) (35.8 km2);

Mean depth—1.4 m from High Bridge near Bulow Creek to south Daytona,

 1.7 m from south Daytona to Ponce De Leon Inlet;

Tidal amplitude—0.7 m;

Area of catchment—~340 mi2 (~880 km2

);

Catchment area:Estuary area ratio—~24:1; and

Average annual rainfall—~50 inches/year over last 30 years. 

Table 3. Land uses in the HRE and watershed (SJRWMD 2004). 

Land Use % Source

Water/Wetland 36% Steward et al. 2010

Range/Forest 34% Steward et al. 2010

Urban 22% Steward et al. 2010

Agriculture 5% Steward et al. 2010

Other 3% Steward et al. 2010
Draft 5 August 2010 

Figure 1. Halifax River Planning Unit, Including the 1998 303(d) list and Planning List waters.
Draft 6 August 2010 

Sources of Nutrients

Potential sources of nutrients in the watershed include stormwater runoff, agriculture, silviculture, 

urban/suburban land uses, atmospheric deposition, sediment loading and turbidity from shoreline erosion,

and septic tanks. 

Septic tanks have been identified as a potential source of nutrients (N and P), pathogens, and other pollutants 

that can pose a threat to public health (SJRWMD 2003). Surface waters can be adversely affected by the direct 

or indirect runoff of septic system leachate. Areas in the Halifax River watershed that have been identified by 

the Volusia County Health Department as having potential septic tank problems include the town of Ormond 

by the Sea, neighborhoods around Ormond Beach along the Tomoka River, Port Orange, Rose Bay, and the 

area along Spruce Creek. Septic tank failure rates in Flagler, St. Johns, and Volusia Counties exceed state 

failure rates (SJRWMD 2003).

A wasteload allocation was established for all the major surface water dischargers into the north Halifax River 

Estuary based on a number of water quality studies that identified significant nutrient problems (Magley 

1990). It was determined that existing point sources of nitrogen and phosphorus were approximately 1.5 to 

2.4 times the nonpoint load. In addition, there were violations of the Class III DO criteria, elevated nutrients 

and chlorophyll a levels, depressed invertebreate populations, and sediments enriched with metals. When 

compared to other estuaries in Florida, the overall trophic state index was considered fair to poor. Based upon 

the potential impacts of further loads, an advanced wastewater treatment (AWT) was implemented for all the 

major dischargers (Magley 1990).

Five permitted domestic WWTFs discharge into the HRE (Figure 1; Table 4): Daytona Beach/Bethune Point (13

million gallons per day [MGD]), city of Holly Hill (2.4 MGD), Ormond Beach (6 MGD), Port Orange (6 MGD), and 

New Smyrna Beach (6 MGD) drain into the HRE watershed (SJRWMD 2006). 

Tables 4a and 4b show sources of nutrients and loads in the north and south HRE, respectively, using a 

Pollution Load Screening Model (PLSM) (Adamus and Bergman 1995). The model’s input requirements are 

spatial data layers (drainage boundary, land use, soil type, and annual rainfall), runoff coefficients (volumetric 

ratio of runoff to total rainfall derived from the literature), the ratio of pervious to impervious surface area for 

a particular land use group, runoff concentrations (literature values derived from field studies), and best 

management practice (BMP) treatment efficiencies (literature values applied to development occurring after 

stormwater rule implementation). Estimated loads from 1983-1986 are included in Table 4a. A comparison of 

2004 WWTF loads to those of 1986 show remarkable declines in TN and TP loads in to the north HRE.

Table 4a. Estimates of current (circa 2004) external loadings of TN and TP to the north HRE. 

g/m2/yr = Grams per square meter of estuary area per year

*SJRWMD PLSM-estimated surface water loadings based on 2004 land use and 2001–08 average annual rainfall, which is nearly equivalent to 30-year 

average annual rainfall. Major tributary streams are the Tomoka River, Bulow Creek, 11th St. Canal, and Reed Canal in the north HRE; Spruce Creek, 

Turnbull Creek, and Halifax Canal in the south HRE.

** Domestic WWTFs: Daytona/Westside, Holly Hill, Ormond Beach in north HRE; Port Orange, New Smyrna Beach (@27% of annual discharge) in south

HRE. WWTF loading estimates calculated from data obtained from FDEP, Tallahassee.. 

***1983-1986 values adapted from Magley 1990.

North HRE Sources TN (kg/yr) TP (kg/yr)

Surface Water Drainage* 207,906 34,088 

Atmospheric Deposition 8,911 203 

WWTFs** (1983-1986)*** 41,015 (317,621) 9,203 (110,750)

Ground Water Unquantified; assumed negligible 

by Scholl et al. 1980

Unquantified; assumed negligible 

by Scholl et al. 1980

Total Loading: 257,832

(13.26 g/m2/yr) 

43,494 

(2.24 g/m2/yr) 
Draft 7 August 2010 

Table 4b. Estimates of current (circa 2004) external loadings of TN and TP to the south HRE. 

*SJRWMD PLSM-estimated surface water loadings based on 2004 land use and 2001–08 average annual rainfall, which is nearly equivalent to 30-year 

average annual rainfall. Major tributary streams are the Tomoka River, Bulow Creek, 11th St. Canal, Reed Canal in the north HRE; Spruce Creek, Turnbull 

Creek, and Halifax Canal in the south HRE.

** Domestic WWTFs: Daytona/Westside, Holly Hill, Ormond Beach in north HRE; Port Orange, New Smyrna Beach (@27% of annual discharge) in south

HRE. WWTF loading estimates calculated from data obtained from FDEP, Tallahassee.

South HRE Sources TN (kg/yr) TP (kg/yr)

Surface Water Drainage* 196,284 32,927 

Atmospheric Deposition 7,516 171 

WWTFs** 17,335 4,492

Ground Water Unquantified; assumed negligible 

by Scholl et al. 1980

Unquantified; assumed negligible 

by Scholl et al. 1980

Total Loading: 221,135

(13.04 g/m2/yr) 

37,590

(2.22 g/m2/yr) 

Biological Summary

The biological communities that characterize the vast majority of the aquatic habitat in the HRE are tidal 

marshes and oyster reefs (Steward et al. 2010). A large portion of the north HRE has been affected by 

historical dredge and fill activities that has limited the extent of the biological communities in this area. In 

addition, there is anecdotal information from District biologists that the few oysters or oyster reefs present in 

the N. HRE do not appear healthy. There are a disproportionate number of undersized or small individuals per 

reef population. There is no evidence to suggest that nutrients are involved in the apparent differences 

between oysters in the north HRE compared to those in the south HRE (Steward personal communication, 

August 2010).

Oyster Reefs

Oyster reefs are a major aquatic community in the HRE and locally decrease turbidity by trapping sediment. 

They present a dynamic, community-forming refugia, nursery grounds, and feeding areas for a variety of other 

estuarine organisms. Oysters provide hard substrate and habitat for many other species, including mud crabs 

(Rhithropanopeus harrisii), blue crabs (Callinectes sapidus), and amphipods. 

Oyster density diminishes considerably near south Daytona and does not increase substantially until the 

Tomoka River area (Steward et al. 2010). Habitat maps suggest a spatial divide in the oyster population 

abundance between Port Orange and Daytona Beach, and between Daytona Beach and Ormond Beach, caused 

primarily by historical dredge-and-fill activities. Approximately 75% of the shoreline in the north HRE is 

hardened, and approximately 50% of the main east and west shorelines in the south HRE are hardened. 

Dredge-and-fill activities are primarily responsible for the disappearance of oysters, not altered water quality, 

including increased nutrients (Steward et al. 2010). 

A three-year study conducted in the nearby Guana Tolomato Matanzas (GTM) Estuary that looked at oyster 

growth rates at two sites with different nutrient loading scenarios found that density, length, and biomass 

increased in oyster communities with higher nutrient loads. This finding suggested that the oyster 

communities were capable of assimilating current nutrient loads and responded positively to increases in 

nutrient loads (at least to concentrations within the range of current levels) (Dix, 2010). 

Salt Marsh

Salt marshes are another common community type in the HRE. Dominant plants include saltmarsh cordgrass 

(Spartina alterniflora), black needlerush (Juncus roemerianus), glasswort (Salicornia spp.), and saltwort (Batis
Draft 8 August 2010 

maritima). This habitat supports a large population of infaunal organisms as well as a variety of transient fish 

and wildlife. 

McFarlin and Brewer (2008) assessed the effect of fertilization on food webs in 19 coastal estuaries in Georgia 

and found that fertilization increased the cover, height, and biomass of Spartina and the height of Juncus, but 

decreased Juncus cover and biomass. The study also showed increases in herbivores (grasshoppers) with 

fertilization, but little to no effect on decomposers (fungi) or detritivores (snails). In comparisons between 

fertilized plots and control plots, variation among control sites was typically greater than the effects of 

fertilization. These results suggest that the effects of increased nutrients at any particular site would be 

difficult to distinguish from the natural variability among sites, and that increased nutrients (within the range 

tested) are likely to have a stimulatory effect on beneficial plants and herbivores.

Plankton

There have been occasional algal blooms on the east coast of Florida, with a Karenia brevis bloom occurring in 

the nearby GTM Estuary in October 2007. The bloom began in Atlantic coastal waters between the mouths of 

the St. Johns and Nassau Rivers. It was transported south by winds and currents, entering both the St. 

Augustine and Matanzas Inlets. Cell counts as high as 5 million cells per liter were recorded in the Matanzas 

River south of the 206 bridge (Hitchcock and Frazel, unpublished raw data). No evidence suggests that 

nearshore nutrients were responsible for the origination or persistence of the bloom.

Water Quality Data Sources and Studies

According to the STORET Watershed Summary for the Upper East Coast, including the Halifax Planning Unit, 

the organizations listed below conduct water quality monitoring. A complete list of monitoring stations is 

available at: http://cfpub.epa.gov/surf/huc.cfm?huc_code=03080201. 

21FLA – Florida Department of Environmental Protection

21FLCEN - Florida Department of Environmental Protection

21FLDOH – Florida Department of Health, Division of Environmental Health, Bureau of Water (Florida)

21FLFMRI - Florida Fish and Wildlife Conservation Commission/Florida Marine Research Institute

21FLGTM - Guana Tolomato Matanzas (GTM) Estuarine (National Estuarine Research Reserve–Florida)

21FLGW - Florida Department of Environmental Protection

21FLKWAT - Florida LakeWatch

21FLMRC - Marine Resources Council of East Florida

21FLSEAS - Florida Department of Environmental Protection

21FLSJWM - St. Johns River Water Management District (Florida)

21FLVEMD - Volusia County Environmental Health Lab (Florida)

21FLWPB - Florida Department of Environmental Protection

Water quality conditions from 1992 to 2008 suggest a division of the HRE system into two or potentially three 

segments. Spatial trends for selected water quality parameters are shown in the following graphs. Clear shifts 

in spatial gradients for salinity and turbidity exist (Figure 2); one shift occurs from the Matanzas system (Sites 

MAT and JXTR26) into the north HRE and the other from the north HRE into the south HRE. 

The spatial breakpoints, particularly between the north and south HRE, occur at tidal null point locations, 

which is indicative of the strong influence of hydrodynamics and the full extent of the daily ocean/estuary 

exchange on the region’s salinity and turbidity. There is no obvious breakpoint between the Tomoka and 

Daytona areas of the north HRE segment. 
Draft 9 August 2010 

A similar spatial pattern exists for total suspended solids (TSS), as lower levels are consistent throughout the 

south HRE compared with the north HRE (Figure 3). However, there is no spatial difference in TSS 

concentrations between the Tomoka and Daytona areas of the north HRE, and between the north HRE and the 

south Matanzas sites (MAT and JXTR26).

Figure 2. Spatial distribution of salinity and turbidity (mean and standard deviation) by sampling site.

0

5

10

15

20

25

Turbidity (ntu)

Site

Turbidity

Dry Season Wet Season Mean 1992-2008 +Std Dev

JXTR26 1997-2008

0

5

10

15

20

25

30

35

40

45

Salinity (ppt)

Site

Salinity

Dry Season Wet Season

Mean 1992-2008 +Std Dev

JXTR26 1997-2008

Tidal null point

N. HRE

Tomoka

N. HRE

Daytona S. HRE
Draft 10 August 2010 

Figure 3. Spatial distribution of TSS (mean and standard deviation) by sampling site.

 

Chlorophyll a, TN, and TP concentrations are clearly higher throughout most of the HRE compared with levels 

in south Matanzas (Figure 4), with mean values 7.9 μg/L, 0.9 mg/L, and 0.17 mg/L, respectively. Within the 

HRE system, spatial differences are not readily apparent (Figure 4); however, nonparametric analyses indicate 

a significant difference between the north and south HRE for chlorophyll a and TN, not for TP (Kruskal-Wallis 

and Mann-Whitney; family α = 0.15 for the 3 segments; individual α = 0.05 for pairs of segments).

Higher TSI values1

The information above regarding the spatial distribution of physical properties, habitat, and water quality 

supports a recommendation for dividing the HRE into two segments, south HRE and north HRE (Tomoka and 

Daytona areas combined), for the development of segment-specific nutrient criteria. 

were calculated for the north HRE than for the south HRE (Figure 4), reflecting differences in

chlorophyll a and nutrient levels. Similarly, TSI values in the south HRE continue to decline as the distance to 

Ponce De Leon Inlet shortens. 

 

1 TSI values were calculated using the equation specified in FDEP (1996), and the value ranges for estuaries, as specified by FDEP (1996), are as follows: 

0–49, good; 50–59, fair; and 60–100, poor.

0

10

20

30

40

50

60

70

TSS (mg/l)

Site

Total Suspended Solids

Dry Season Wet Season Mean 1992-2008 +Std Dev

JXTR26 1997-2008 N. HRE

Tomoka

N. HRE

Daytona S. HRE

Tidal null point
Draft 11 August 2010 

Figure 4. Spatial distribution of chlorophyll a, TN, and TP (mean and standard deviation) by sampling site.

0

5

10

15

20

25

Chl a Corr (μg/L)

Site

Chlorophyll a Corrected

Dry Season Wet Season Mean 1992-2008 +Std Dev

JXTR26 1997-2008

0

0.5

1

1.5

2

2.5

TN (mg/l)

Site

Total Nitrogen

Dry Season Wet Season

JXTR26 1997-2008

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

TP (mg/l)

Site

Total Phosphorus

Dry Season Wet Season Mean 1992-2008 +Std Dev

JXTR26 1997-2008

N. HRE

Tomoka

N. HRE

Daytona S. HRE

Tidal null point

---- N. HRE ---- ---- S. HRE ----
Draft 12 August 2010 

TSI values throughout the HRE have decreased over time, as shown in the comparison of mean TSI values 

between the 1992–2000 and 2000–08 periods (Figure 5). The more recent TSI values strongly suggest upper 

mesotrophic conditions throughout the north HRE, and lower mesotrophic to oligotrophic conditions in the 

south HRE. 

Figure 5. Spatial distribution of TSI values (means) for two periods, 1992–2000 and 2000–08.

Temporal Considerations and the Reference Period Method 

The decrease in trophic state from one decade to the next is a consequence of general reductions in 

chlorophyll a since 2000, a likely response to the precipitous decline in TN over the same period (Figure 6). TN 

started to decline in the early 1990s, reaching much lower and more stable levels from 1996 to 2008. While 

other unknown factors may have contributed to the reduction in TN concentrations, reductions in effluent 

loadings from domestic WWTFs occurred during the same period (D. Judy, FDEP Central District, pers. com., 

March 2010). 

TP does not follow a similar trend. The temporal trend in TP is flat over the full period of record. It is likely 

that nonpoint sources and internal processes are more influential in controlling TP than WWTF loadings. 

0

10

20

30

40

50

60

TSI

Trophic State Index 1992 - 2000

0

5

10

15

20

25

30

35

40

45

50 2000 - 2008

TSI

N. HRE

Tomoka

N. HRE

Daytona

S. HRE
Draft 13 August 2010 

Because TN:TP (and dissolved inorganic nitrogen:dissolved inorganic phosphorus [DIN:DIP]) ratios are less 

than 5 (by weight) and chlorophyll a appears to be more responsive to TN than to TP, the HRE system overall 

may be N limited. Data collected by the University of Florida also support N limitation, but this 

characterization should be treated with caution because no bioassay analyses have been performed for 

verification (E. Phlips, University of Florida, pers. com., February 2010).

Figure 6. Temporal trends in TN, TP, and chlorophyll a for the HRE overall, 1992–2008.

0

0.5

1

1.5

2

2.5

3

TN (mg/l)

Year

Total Nitrogen

Dry Season Wet Season

HL01-HL04

Low and stable in HRE 

overall since 1996

However, [TP ] trend was flat: 

0.14 – 0.17 mg/L long-term avg.

0

2

4

6

8

10

12

14

16

18

20

Chl a corr (μg/l)

Year

Chlorophyll a Corrected

Dry Season Wet Season

HL12-HL20

Possible response in Chla to TN 

Wet season spikes >11 μg/L reduction since wet season 2000

persist; concurrent with 

[TN] & [TP] increases, esp. 

in Aug. – Sept.
Draft 14 August 2010 

Waters on the 303(d) List 

Table 5 provides the complete list of waters that are verified impaired in the Upper East Coast–Halifax River Planning Unit, and Table 6 identifies the 

waters that are proposed for delisting. 

Table 5. Waterbodies on the 2010 Verified List for impairments in the Halifax River Planning Unit (FDEP).

3M = Class III Marine

Basin 

Group 

Name

Planning 

Unit

Waterbody 

ID (WBID)

Waterbody 

Segment 

Waterbody 

Type

Waterbody 

Class1

Parameters 

Assessed 

Using 2001 

IWR

Concentration 

Causing 

Impairment2

Priority for 

TMDL 

Development3

Comments 

(# Exceedances/# 

Samples) 

PP = Planning Period VP 

= Verified Period4

Upper 

East Coast

Halifax 

River 

Unit 8117B Dunlawton Beach 3M

Coliforms 

(Beach 

Advisory)

> 21 days of 

advisories in a 

year High

Beach advisories posted 

for a total of 36 days in 

2003.

Upper 

East Coast

Halifax 

River 

Unit 8117C Hilton Beach 3M

Coliforms 

(Beach 

Advisory)

> 21 days of 

advisories in a 

year High

Beach advisories posted 

for a total of 73 days in 

2003 and 23 days in 

2004.

Upper 

East Coast

Halifax 

River 

Unit 8117D Silver Beach Beach 3M

Coliforms 

(Beach 

Advisory)

> 21 days of 

advisories in a 

year High

Beach advisories posted 

for a total of 33 days in 

2003.

Upper 

East Coast

Halifax 

River 

Unit 2363A Halifax River Estuary 3M

Mercury (In 

Fish Tissue)

> 0.43 mg/kg in 

Fish tissue High

Listed based on king 

mackerel and bull shark 

concentrations exceeding 

0.43 mg/kg; collected in 

2002-2004. 

Upper 

East Coast

Halifax 

River 

Unit 2363B Halifax River Estuary 3M Copper >3.7 μg/L High pp = 22/48; vp = 9/50

Upper 

East Coast

Halifax 

River 

Unit 2363B Halifax River Estuary 3M Iron > 0.3 mg/L High

PP - 39/51 VP - 46/65 

May be due to high iron 

concentrations in 

groundwater. Prelimary 

investigation reveals that 

much of the data from 

surrounding monitoring 

wells significantly exceed 

the 0.3 mg/L criterion 

(see the groundwater 

chapter in the "Upper 

East Coast Basin Status 

Report"). Will be 

investigated further. 
Draft 15 August 2010 

Basin 

Group 

Name

Planning 

Unit

Waterbody 

ID (WBID)

Waterbody 

Segment 

Waterbody 

Type

Waterbody 

Class1

Parameters 

Assessed 

Using 2001 

IWR

Concentration 

Causing 

Impairment2

Priority for 

TMDL 

Development3

Comments 

(# Exceedances/# 

Samples) 

PP = Planning Period VP 

= Verified Period4

Upper 

East Coast

Halifax 

River 

Unit 2363B Halifax River Estuary 3M

Mercury (In 

Fish Tissue)

> 0.43 mg/kg in 

Fish tissue High

Listed based on king 

mackerel and bull shark 

concentrations exceeding 

0.43 mg/kg; collected in 

2002-2004. 

Upper 

East Coast

Halifax 

River 

Unit 2363J Palm Coast Estuary 3M

Mercury (In 

Fish Tissue)

> 0.43 mg/kg in 

Fish tissue High

Listed based on king 

mackerel and bull shark 

concentrations exceeding 

0.43 mg/kg; collected in 

2002-2004. 

Upper 

East Coast

Halifax 

River 

Unit 2672 Rose Bay Estuary 3M

Dissolved 

Oxygen < 4.0 mg/L High

PP - 1/49 VP -13/43 DO 

met verification 

threshold of IWR, and TN 

was identifed as a 

causative pollutant. PP 

median TN = 0.66 mg/L 

(n=153); median TP = 

0.13 mg/L (n=154); no 

BOD data. VP median TN 

= 1.115 mg/L (n=17); 

median TP =0.14 mg/L 

(n=17); BOD = 2.05 

(n=16).

Upper 

East Coast

Halifax 

River 

Unit 2672 Rose Bay Estuary 3M

Mercury (In 

Fish Tissue)

> 0.43 mg/kg in 

Fish tissue High

Listed based on king 

mackerel and bull shark 

concentrations exceeding 

0.43 mg/kg; collected in 

2002-2004. 

Upper 

East Coast

Halifax 

River 

Unit 2674A Spruce Creek Estuary 3M Copper > 3.7 μg/L Medium pp = 6/9; vp = 6/20

Upper 

East Coast

Halifax 

River 

Unit 2674A Spruce Creek Estuary 3M

Dissolved 

Oxygen < 4.0 mg/L High

pp = 96/285; vp = 40/137 

DO met verification 

threshold of IWR and 

linked to nutrients,
Draft 16 August 2010 

Basin 

Group 

Name

Planning 

Unit

Waterbody 

ID (WBID)

Waterbody 

Segment 

Waterbody 

Type

Waterbody 

Class1

Parameters 

Assessed 

Using 2001 

IWR

Concentration 

Causing 

Impairment2

Priority for 

TMDL 

Development3

Comments 

(# Exceedances/# 

Samples) 

PP = Planning Period 

VP = Verified Period4

Upper 

East Coast

Halifax 

River 

Unit 2674A Spruce Creek Estuary 3M

Fecal 

Coliform

> 400 

Colonies/100 mL Low

pp = 69/305; vp = 

33/123 

Upper 

East Coast

Halifax 

River 

Unit 2674A Spruce Creek Estuary 3M

Mercury (In 

Fish Tissue)

> 0.43 mg/kg in 

Fish tissue High

Listed based on king 

mackerel and bull shark 

concentrations 

exceeding 0.43 mg/kg; 

collected in 2002-2004. 

Upper 

East Coast

Halifax 

River 

Unit 2674A Spruce Creek Estuary 3M

Nutrients 

(Chla)

Median TN = 0.85 

mg/L Median TP 

= 0.150 mg/L High

Annual averages for 

1996 and 2003 exceed 

the threshold of 11 

μg/L for estuaries. 

Annual averages: 1994 - 

6.04 μg/L; 1995 - 5.46 

μg/L; 1996 - 11.74 μg/L; 

1997 - 4.16 μg/L; 1998 - 

9.80 μg/L; 1999 - 5.30 

μg/L; 2000 - 3.82 μg/L; 

2001 - 5.96 μg/L; 2003 - 

12.05 μg/L; 2005 - 2.95 

μg/L. Nutrients (Chla) 

Nitrogen was identified 

as the limiting nutrient 

based on 80% of the 

TN/TP ratios were less 

than 10.

Upper 

East Coast

Halifax 

River 

Unit 2674B

Strickland Bay 

At Us1 Estuary 3M

Fecal 

Coliform > 400 mg/L Low pp = 0/1; vp = 9/24

Upper 

East Coast

Halifax 

River 

Unit 2674B

Strickland Bay 

At Us1 Estuary 3M

Mercury (In 

Fish Tissue)

> 0.43 mg/kg in 

Fish tissue High

Listed based on king 

mackerel and bull shark 

concentrations 

exceeding 0.43 mg/kg; 

collected in 2002-2004. 

Upper 

East Coast

Halifax 

River 

Unit 2363C Tomoka Basin Estuary 3M

Mercury (In 

Fish Tissue)

> 0.43 mg/kg in 

Fish tissue High

Listed based on king 

mackerel and bull shark 

concentrations 

exceeding 0.43 mg/kg; 

collected in 2002-2004. 
Draft 17 August 2010 

Basin 

Group 

Name

Planning 

Unit

Waterbody 

ID (WBID)

Waterbody 

Segment 

Waterbody 

Type

Waterbody 

Class1

Parameters 

Assessed 

Using 2001 

IWR

Concentration 

Causing 

Impairment2

Priority for 

TMDL 

Development3

Comments 

(# Exceedances/# 

Samples) 

PP = Planning Period 

VP = Verified Period4

Upper 

East Coast

Halifax 

River 

Unit 2634A Tomoka River Estuary 3M

Mercury (In 

Fish Tissue)

> 0.43 mg/kg in 

Fish tissue High

Listed based on king 

mackerel and bull shark 

concentrations 

exceeding 0.43 mg/kg; 

collected in 2002-2004. 

Upper 

East Coast

Halifax 

River 

Unit 2678 Turnbull Bay Estuary 3M

Dissolved 

Oxygen < 4.0 mg/L Medium

PP - 10/13 VP - 12/45 

DO met verification 

threshold of IWR, TN 

and BOD were 

identified as causative 

pollutants. PP median 

TN = 0.996 mg/L (n=8), 

median TP = 0.1925 

mg/L (n=12), no BOD 

data. VP median TN = 

1.295 mg/L (n=12), 

median TP = 0.1755 

mg/L (n=16), median 

BOD = 2.5 mg/L (n=25). 

Upper 

East Coast

Halifax 

River 

Unit 2678 Turnbull Bay Estuary 3M

Mercury (In 

Fish Tissue)

> 0.43 mg/kg in 

Fish tissue High

Listed based on king 

mackerel and bull shark 

concentrations 

exceeding 0.43 mg/kg; 

collected in 2002-2004. 

Upper 

East Coast

Halifax 

River 

Unit 2678 Turnbull Bay Estuary 3M

Nutrients 

(Chla)

Median TN = 1.00 

mg/L Median TP 

= 0.192 mg/L Medium

Annual average for 

2003 exceeds the 11 

μg/L threshold for 

estuaries. Only annual 

average is from 2003 -

17.17 μg/L. Nitrogen 

identified as the 

limiting nutrient based 

on all the TN/TP ratios 

less than 10. 

Upper 

East Coast

Halifax 

River 

Unit 2642

Unnamed 

Branch Estuary 3M

Mercury (In 

Fish Tissue)

> 0.43 mg/kg in 

Fish tissue High

Listed based on king 

mackerel and bull shark 

concentrations 

exceeding 0.43 mg/kg; 

collected in 2002-2004. 
Draft 18 August 2010 

Table 6. Estuarine waterbodies in the Halifax River Planning Unit that are proposed for delisting (FDEP).

3M = Class III Marine

Basin 

Group 

Name

Planning 

Unit

WBID Waterbody 

Segment

Waterbody 

Type

Waterbody 

Class

1998 303(D) 

Parameters of 

Concern

Parameters 

Evaluated Using 

IWR

Comments (with # of Exceedances/# of 

Samples)

Upper 

East 

Coast

Halifax 

River

2363A Halifax 

River

Estuary 3M Coliforms Fecal Coliform PP - 3/532 VP - 0/202 

Upper 

East 

Coast

Halifax 

River

2363B Halifax 

River

Estuary 3M Lead Lead PP -2/47 VP - 4/31 

Upper 

East 

Coast

Halifax 

River

2363B Halifax 

River

Estuary 3M Nutrients Nutrients (Chla) No annual average exceeds the 11 μg/L 

threshold for estuaries. Annual averages: 

1994 - 10.30 μg/L, 1995 - 5.94 μg/L, 1996 

- 6.45 μg/L, 1997 - 6.84 μg/L, 1998 - 7.07 

μg/L, 1999 - 9.40 μg/L, 2000 - 7.72 μg/L, 

2001 - 5.68 μg/L, 2002 - 8.01 μg/L, 2003 - 

5.36 μg/L, 2004 - 5.74 μg/L, 2005 - 3.79 

μg/L. Assessment category will be 3b 

because biology or site-specific data is 

needed to validate non-impairment 

assessment.

Upper 

East

Coast

Halifax 

River

2672 Rose Bay Estuary 3M Coliforms Fecal Coliform PP -8/57 VP - 1/39

Upper 

East 

Coast

Halifax 

River

2674A Spruce 

Creek

Estuary 3M Iron Iron PP - 0/0 VP - 22/22 A document has 

been prepared that supports that 

elevated iron concentrations are a 

natural condition. Consequently, it has 

been assigned an EPA Integrated Report 

Category 4C. 

Upper 

East 

Coast

Halifax 

River

2634A Tomoka 

River

Estuary 3M Nutrients Nutrients (Chla) No annual average exceeds the 11 μg/L 

threshold for estuaries. Annual averages: 

1994 - 9.50 μg/L, 1995 - 8.26 μg/L, 1996 - 

5.79 μg/L, 1997 - 4.30 μg/L, 1998 - 5.41 

μg/L, 1999 - 6.34 μg/L, 2000 - 7.41 μg/L, 

2001 - 7.96 μg/L, 2002 - 4.47 μg/L, 2003 - 

7.91 μg/L, 2004 - 7.15 μg/L, 2005 - 4.38 

μg/L.

Upper 

East 

Coast

Halifax 

River

2634A Tomoka 

River

Estuary 3M Iron Iron PP - 43/54 . VP - 53/62 A document has 

been prepared that supports that 

elevated iron concentrations are a 

natural condition. Consequently, it has 

been assigned an EPA Integrated Report 

Category 4C. 
Draft 19 August 2010 

Basin 

Group 

Name

Planning 

Unit

WBID Waterbody 

Segment

Waterbody 

Type

Waterbody 

Class

1998 303(D) 

Parameters of 

Concern

Parameters 

Evaluated Using 

IWR

Comments (with # of Exceedances/# of 

Samples)

Upper 

East 

Coast

Halifax 

River

2674 Spruce 

Creek

Stream 3F Dissolved Oxygen Dissolved Oxygen PP - 9/52 VP - 9/59 

Upper 

East 

Coast

Halifax 

River

2674 Spruce 

Creek

Stream 3F Nutrients Nutrients (Chla 

and Histchla)

Annual average for 1997 exceeds the 

threshold of 20 μg/L for streams. Annual 

averages: 1995 - 8.48 ug/L; 1996 - 1.08 

μg/L, 1997 - 21.22 ug/L; 1998 - 9.06 ug/L; 

1999 - 1.57 μg/L, 2000 - 2.82 μg/L, 2001- 

7.56 ug/L; 2002 - 1.25 μg/L; 2003 - 2.19 

μg/L; 2005 - 2.80 μg/L. Nutrients (Chla) 

Delist since the annual for every year 

after 1997 has been less than the 

threshold. The historic minimum annual 

chlorophyll average of 10.19 ug/L was not 

exceeded by more than 50% for two 

consecutive years.

Upper 

East 

Coast

Halifax 

River

2674 Spruce 

Creek

Stream 3F Iron Iron PP - 45/52 . VP - 51/57 A document has 

been prepared that supports that 

elevated iron concentrations are a 

natural condition. Consequently, it has 

been assigned an EPA Integrated Report 

Category 4C. 

Upper 

East 

Coast

Halifax 

River

2634 Tomoka 

River

Stream 3F Coliforms Fecal Coliform PP - 41/267 VP - 20/150 

Upper 

East 

Coast

Halifax 

River

2634 Tomoka 

River

Stream 3F Iron Iron PP - 13/31 . VP - 2/21 

Upper 

East 

Coast

Halifax 

River

2634 Tomoka 

River

Stream 3F Nutrients Nutrients (Chla) No annual average exceeds the 20 μg/L 

threshold for streams. Annual averages 

are: 1994 - 4.25 μg/L,1995 - 3.34, 1996 - 

2.32 μg/L, 1997 - 2.26 μg/L, 1998 - 2.49 

μg/L, 1999 - 3.69 μg/L, 2000 - 2.65 μg/L, 

2001 - 1.78 μg/L, 2002 - 1.88 μg/L, 2003 - 

2.20 μg/L, 2004 - 1.55 μg/L. Assessment 

category will be 3b because biology or 

site-specific data is needed to validate 

non-impairment assessment.
Draft 20 August 2010 

Numeric Nutrient Criteria Recommendation 

The analyses presented in this report are based on ambient data collected as part of long-term, routine

monitoring networks designed to address basic status and trend questions related to trophic state and sitespecific nutrient targets. Therefore, short-term, high-frequency data (event or experimental data), permit 

data, episodic data, permit compliance data, facility status or compliance data, volunteer network data, or 

data of unknown quality were not included (J. Steward, pers. comm., 2010).

With respect to the HRE, as well as the other bar-built, lagoonal estuaries oriented linearly along Florida’s east 

coast, the development of nutrient criteria should consider at least three points:

1. Spatial heterogeneity within the system, segmenting the system accordingly for the 

purpose of setting segment-specific criteria;

2. Temporal shifts in water quality by which a certain period of record representing desirable 

trophic water quality conditions (when designated uses are fully met) can be selected to 

develop protective criteria; and

3. Multiple lines of evidence, helping to ensure a high level of certainty or confidence 

associated with the nutrient criteria derived from any one or all lines of evidence. 

The following lines of evidence are used for the HRE: 

• A reference period method; 

• Chlorophyll a–nutrient regression analyses; and

• Two empirical models that pertain to estuaries in general. 

The primary source for the site-specific data required by these methods is the VCEMD, and the full period of 

record for these data extends from 1992 to 2008.

Reference Period

For the reference period method, a reference period (preferably 5 consecutive years as a minimum) within the 

full period of record that provided the lowest chlorophyll a and nutrient levels along with a desirable trophic 

state was selected to calculate annual and wet-season median concentrations of chlorophyll a, TN, and TP. A 

trophic condition was considered desirable for a given segment if the average of TSI values, as applied to 

estuaries (FDEP 1996), was less than 50. Note that this target has not been fully evaluated for all estuary 

systems, and it may need to be shifted depending on the biological communities in a given estuary. Because 

nutrient loadings and concentrations (and presumably chlorophyll a and TSI) in the HRE are largely a function 

of watershed rainfall–runoff, the selected reference period should not be a drought period but, rather, one 

that approximates the 30-year annual average rainfall for the region.

Chlorophyll a/[TN] or [TP] Regression Models

All data from the full period of record were reduced and organized into annual average, average monthly, and 

average monthly wet-season (peak production months of July through September only) datasets. Simple 

linear regression models were developed per segment from these reduced datasets: [chlorophyll a] regressed 

against [TN] and against [TP]. If the regressions had moderate to strong correlation (r) and significance (p) 

statistics, they were used to predict median concentration limits based on reference period [chlorophyll a] 

medians (the de facto chlorophyll a target levels). Multivariate regression analysis was also performed to 
Draft 21 August 2010 

identify major variables that may regulate algal production (i.e., chlorophyll a levels) in the HRE and, if 

statistically feasible, could be used as a complementary line of evidence to help set nutrient criteria.

General Models

Two general nutrient models were applied to the HRE to predict the limits or acceptable ranges of nutrient 

loadings and concentrations related to a desirable trophic state (e.g., mesotrophy or oligotrophy). TP limits 

were derived by the Florida model, an empirically defined relationship between hydraulic loading (mean 

depth/water residence time) and nutrient loading limits for mesotrophic Florida systems (Steward and Lowe 

2010). Because the Florida TP loading limit model (Steward and Lowe 2010) is consistent with the 

Vollenweider (1976) TP critical loading model, the Vollenweider model equation was adjusted (specifically the 

τw exponent) to fit the Florida model (Steward and Lowe 2010), which was used to predict both the upper and 

lower mesotrophic [TP] and an oligotrophic level of [TP]:

Upper mesotrophic [TP] = LM(P) X τw

0.847/z(1 + τw0.5), which was empirically derived;

Lower mesotrophic [TP] = LM(P) X τw

0.7/z(1 + τw0.5) as a theoretical derivation;

Oligotrophic [TP] = LO(P) X τw

0.5/z(1 + τw0.5) as a theoretical derivation; whereby LM, LO = mesotrophic, 

oligotrophic loads (g/m2

/yr); τw = water residence time; z= mean depth (m).

The N-analog of the Vollenweider (1976) model was not equivalent to or consistent with the Florida TN trophic 

loading model (Steward and Lowe 2010), and thus the Vollenweider model could not be used to predict [TN]. 

Therefore, a second model (Dettmann 2001) for estuaries was selected to predict [TN] based on a trophically 

desirable TN loading limit as determined by the prior application of the Florida model. The Dettmann (2001) 

model equation is:

[TN] = (LN X τw / V + [TN]s)(1/1 + ατw); whereby LN = TN loading (g/yr), τw = freshwater residence time 

(yr), V = volume of estuary (liters, L) segment, [TN]s = average [TN] at segment boundaries (g/L), α = 

1st order rate coefficient for N loss (3.6 yr-1).

Among the three lines of evidence, the reference period provides the primary line of evidence because it 

constitutes the most direct analysis of site-specific data that represents a time when trophic state was

desirable. The other two lines of evidence are model predictions (incorporating site-specific data) and 

regarded as supporting evidence. 

Results and Discussion

Spatial Considerations and Segmentation 

The northern half of the HRE system, including the estuary and catchment, is distinct from the southern half 

based on a number of characteristics. For example, even though the percent proportions of land uses in the 

area are nearly equivalent between the south HRE (S. HRE) and north HRE (N. HRE) (Figure 7; SJRWMD 

geographic information system [GIS] file data on 2004 land use), their population densities are quite different. 

The municipal population density in the north HRE, about 3,300 people/mi2

, is about 1.7 times greater than in 

the south HRE, with about 2,000 people/mi2 (Volusia County Department of Economic Development 2009).
Draft 22 August 2010 

Figure 7. Percent composition of major land uses in the south and north HRE (based on 2004 imagery).

Hydrodynamic properties probably best describe the estuarine differences between the north and south HRE. 

John and Morris (1999) conducted a tracer study with a calibrated RMA2 model (U.S. Army Corps of Engineers) 

to determine two tidal null or node points within the HRE. A tidal node point is a general location within the 

estuary where there is no net water flow in either direction (in this case, north or south). One node point is 

located between south Daytona and Daytona Beach (Figure 8a), and the other is located just south of High 

Bridge and east of the lower reach of Bulow Creek (Figure 8b).

Figure 8a. Location of tidal node at boundary between the north and south HRE.

Agriculture 

Other 

Range/Forest 

Residential 

Urban 

Water/Wetlands 

7%

2%

33%

14%

10%

35%

3% 3%

36%

11% 11%

37% N. HRE

(S. Daytona to Bulow)

S. HRE

(Ponce De Leon Inlet to S. Daytona)

N. HRE

Tidal 

node
Draft 23 August 2010 

Figure 8b. Location of tidal node at boundary between the HRE and Matanzas River Estuary near High 

Bridge.

The Daytona tidal node point marks the initial delineation between the south HRE and north HRE segments. 

The High Bridge node point generally marks the divide between the HRE and Matanzas River Estuary systems. 

The south HRE and north HRE share the same tidal height or amplitude of about 0.7 m and are only slightly 

different in mean depth: 1.7 m and 1.4 m, respectively. However, the flushing rates or water residence times 

between the 2 segments are very different. The north HRE has an approximate freshwater residence time of 

14 days,

2

The spatial distributions of major habitat communities in the HRE estuary and catchment appear to support 

the delineation between the south and north HRE, as described above. Two biological resource maps are 

shown (Figure 9) as evidence of this support. One map shows the distribution of oyster reefs (SJRWMD 2006), 

and the other shows the distribution of aquatic and terrestrial habitat deemed regionally significant (Johnson 

et al. 1998). Oyster reef abundance in the south HRE is clearly contrasted with the near absence of oyster 

reefs in the north HRE (primarily due to dredge-and-fill development during the first 60 years of the 20th

century). The presence of regionally significant habitat is comparatively sparse in the north HRE, especially 

along the estuary, compared with more the intact and contiguous habitats found in the south HRE and in the 

southern portion of the Matanzas River estuary.

while the freshwater residence in the south HRE is 3 days (derived from Sheng et al. 2008), 

attributable to that segment’s proximity to Ponce De Leon Inlet. As shown below, water residence time is an 

important factor that influences the critical nutrient loading limit required to achieve or maintain a desired 

trophic condition. 

 

2 Revised estimate that replaces the 20-day residence time presented at the March 2010 FDEP workshop

Tidal node

Bulow Cr.

Google Earth

High Bridge

N. HRE
Draft 24 August 2010 

Figure 9. Distribution of oyster and other significant habitat communities in the HRE.

Water quality conditions over the period of record also suggest a division of the HRE system into two or 

potentially three segments. The network of VCEMD water quality sites, and their respective data, were 

spatially aggregated into three segments, with the first two segments divided at the north HRE and south HRE 

boundary, as previously described, and a third segment consisting of sites selected to represent the 

northernmost Tomoka area of the north HRE (Figure 10). With an emphasis on those three segments, the 

following graphs show spatial trends for selected water quality parameters. 

The 2000 through 2008 period3

 

3 The reference period changed to 2000–08 from 1996–2008. The latter period was presented as the reference period, along with related results, at the 

March 2010 FDEP workshop.

was selected as a reference period for nutrient criteria development because 

(1) TN levels were about half the levels measured during the early 1990s; (2) chlorophyll a levels dropped since 

the late 1990s, probably in response to the TN reduction, and are consistent with the TMDL-related 

chlorophyll a targets established for other Florida estuaries (typically ≤7 μg/L annual average; Steward and 

Lowe 2010); and (3) trophic status was “good”—upper mesotrophic in the north HRE, and mesotrophic tending 

toward oligotrophic in the south HRE. The TP trend was stable throughout the full period of record (1992–

2008) and likely inconsequential with respect to the estuary’s trophic condition.

SJRWMD Oyster Reef Imagery (2006) SJRWMD NCB Recon Report 

(Johnson et al. 1998)

Regionally Significant 

Habitat (includes salt 

marsh, oyster habitat as 

well as Fw and upland 

habitats)

Oyster reefs

Port 

Orange

Daytona 

Beach

S. HRE

N. HRE

N. HRE

S. HRE
Draft 25 August 2010 

Figure 10. VCEMD water quality sampling sites. Only data from sites HL01 through HL20 were analyzed for 

development of estuarine nutrient criteria for the Halifax River Estuary.

The reference period results for chlorophyll a, TN, and TP are reported as annual median concentrations and 

maximum wet-season median concentrations (based on the highest monthly values, typically observed in July, 

August, and September). These results (Tables 7a and 7b) constitute the first and primary line of evidence in 

the development of nutrient criteria for the north and south HRE. 

Table 7a. HRE reference period results—first line of evidence: Annual median, 2000–08

Segment Chlorophyll a 

(μg/L)

TN 

(mg/L)

TP 

(mg/L)

North HRE 4.5 0.61 0.12

South HRE 3.5 0.39 0.12

Sites Segment

HL01-04 N. HRE/Tomoka

HL05-11 N. HRE/Daytona

HL12-20 S. HRE
Draft 26 August 2010 

Table 7b. HRE reference period results—first line of evidence: Maximum wet-season median, 2000–08 

(July–September)

Segment Chlorophyll a 

(μg/L)

TN 

(mg/L)

TP 

(mg/L)

North HRE 7.9 0.90 0.18

South HRE 5.8 0.51 0.19

The maximum wet-season medians for chlorophyll a (7.9 and 5.8 μg/L for the north HRE and south HRE, 

respectively) are well below the tentative indicator level of 12 μg/L that corresponds to dinoflagellate blooms 

or dominance as indicated by analysis of a U.F., Edward Phlips 2000–01 dataset for the HRE (Steward et al. 

2010). One of the dominant dinoflagellate species in the HRE is Prorocentrum minimum, which, in high bloom 

concentrations, was responsible for oyster mortality in the Chesapeake Bay (National Oceanic and 

Atmospheric Administration [NOAA] 2007). It is not known whether P. minimum densities have reached lethal 

levels in the HRE, and archived samples remain to be analyzed (E. Phlips, University of Florida, pers. com., 

February 2010). 

Chlorophyll a–[TN], [TP] Regression Models 

A covariant trend among average monthly TN, TP, and chlorophyll a was apparent (Figure 11) and prompted 

the development of simple regression models, as described in the section on General Considerations and 

Methods. 

Figure 11. Average monthly TN, chlorophyll a, and TP in the HRE overall. Monthly rainfall data are shown 

for comparison. The 1992–2008 period was used for both water quality and rainfall data.

If any of the regressions proved statistically significant, they were applied toward setting median TN and TP 

based on chlorophyll a “targets” predetermined by the reference period method. The annual average 

regressions were used to predict annual median concentrations. The average monthly regressions were used 

to predict maximum wet-season median limits using either the average July–September datasets or the allmonths datasets, depending on which datasets yielded the better statistical result (r and p values). The results 

of the simple linear regressions are the second line of evidence (Tables 8a and 8b). 

TP, mg/L x 10 

TN, mg/L x 10 

2

4

6

8

10

12

14

Chl a, μg/L

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

2.5” 2.6” 4.3” 1.7” 1.8” 6.9” 6.2” 7.0” 7.7” 5.1” 2.9” 2.6”

rainfall
Draft 27 August 2010 

Table 8a. HRE linear regression results–second line of evidence: Annual median (1992–2008; annual 

average data)

Segment Target 

Chlorophyll a

(μg/L)

TN 

(mg/L)

r p TP 

(mg/L)

r p 

North HRE 4.5 0.58 0.62 .008 Not 

significant

Not 

significant

Not 

significant

South HRE 3.5 0.30 0.74 .0008 Not 

significant

Not 

significant

Not 

significant

Table 8b. HRE linear regression results–second line of evidence: Maximum wet-season median (1992–2008; 

average monthly; July–September only for TN; all months for TP)

Segment Target 

Chlorophyll a

(μg/L)

TN 

(mg/L)

r p TP 

(mg/L)

r p 

North HRE 7.9 0.90 0.69 .004 0.17 0.87 .0002

South HRE 5.8 0.41 0.76 .001 0.15 0.86 .0003

Multivariate analysis (stepwise regression) revealed that the major factor regulating chlorophyll a is water 

temperature, reflecting elevated chlorophyll a in summer (wet-season) months. Incorporating TN and TP 

together or separately did not strengthen the multivariate model. This lack of an enhanced relationship by 

nutrients does not necessarily reflect the system’s nutrient or trophic state (Borum 1996; Cloern 1982). Even if 

a system is nutrient overenriched, its trophic response could be limited by other factors. Given that the HRE 

system has relatively low chlorophyll a and trophic state, other factors controlling chlorophyll a may be 

involved besides nutrients and water temperature. Metrics related to water transparency or light 

availability—Secchi depth and turbidity—do not improve the multivariate relationship either, but that is more 

a consequence of low temporal variability, not magnitude. In fact, turbidities are comparatively and 

persistently high in the south and north HRE. HRE turbidities are typically over twice those observed in the 

Indian River Lagoon (IRL) system (Figure 12). 

High turbidities in the HRE are probably a natural condition. The HRE is similar to many well-flushed, yet 

naturally turbid estuaries in the South Atlantic Bight region (northeast Florida through South Carolina) that are 

dominated ecologically by oyster and marsh communities. A high level of turbidity or tripton is one major 

factor (short water residence time, appreciable tidal amplitude, and tidal current being others) that constrains 

phytoplankton and submersed macrophyte production in favor of marsh production in the estuaries of 

northeast Florida, Georgia, and South Carolina (Lee and Jones 1981; Dame et al. 2000; Roman et al. 2000; 

R. Virnstein, Seagrass Ecosystems Analyst, pers. com., April 2010; E. Phlips, University of Florida, pers. com., 

2010). 

 
Draft 28 August 2010 

Figure 12. Mean turbidity by year (1992–2008) for the north and south HRE compared with the IRL.

General Models 

Both models (Steward and Lowe 2010; Dettmann 2001) require estimates of water residence time and 

external nutrient loadings to predict trophic state, loading limits, and complementary concentrations of TN and 

TP. Tables 9a and 9b present estimates of external nutrient loadings for the north and south HRE, respectively. 

Table 9a. Estimates of current (circa 2004) external loadings of TN and TP to the north HRE. 

g/m2/yr = Grams per square meter per year

*SJRWMD PLSM-estimated surface water loadings based on 2004 land use and 2001–08 average annual rainfall, which is nearly equivalent to 30-year 

average annual rainfall. Major tributary streams are the Tomoka River, Bulow Creek, 11th St. Canal, and Reed Canal in the north HRE; Spruce Creek, 

Turnbull Creek, and Halifax Canal in the south HRE.

** Domestic WWTFs: Daytona/Westside, Holly Hill, Ormond Beach in north HRE; Port Orange, New Smyrna Beach (@27% of annual discharge) in the 

south HRE. WWTF loading estimates calculated from data obtained from FDEP, Tallahassee.

North HRE Sources TN 

(kg/yr)

TP 

(kg/yr)

Surface Water Drainage* 207,906 34,088 

Atmospheric Deposition 8,911 203 

WWTFs** 41,015 9,203

Ground Water Unquantified; assumed negligible 

by Scholl et al. 1980

Unquantified; assumed negligible 

by Scholl et al. 1980

Total Loading: 257,832

(13.26 g/m2/yr) 

43,494 

(2.24 g/m2/yr) 

0

2

4

6

8

10

12

14

16

18

Turbidity (ntu)

Year

Mean Turbidity in the HRE and IRL

Mean Turb. - N. HRE Mean Turb. - S.HRE Mean Turb. - IRL

N. HRE

S. HRE

IRL
Draft 29 August 2010 

Table 9b. Estimates of current (circa 2004) external loadings of TN and TP to the south HRE. 

*SJRWMD PLSM-estimated surface water loadings based on 2004 land use and 2001–08 average annual rainfall, which is nearly equivalent to 30-year 

average annual rainfall. Major tributary streams are the Tomoka River, Bulow Creek, 11th St. Canal, and Reed Canal in the north HRE; Spruce Creek, 

Turnbull Creek, and Halifax Canal in the south HRE.

** Domestic WWTFs: Daytona/Westside, Holly Hill, Ormond Beach in the north HRE; Port Orange, New Smyrna Beach (@27% of annual discharge) in 

the south HRE. WWTF loading estimates calculated from data obtained from FDEP, Tallahassee.

South HRE Sources TN 

(kg/yr)

TP 

(kg/yr)

Surface Water Drainage* 196,284 32,927 

Atmospheric Deposition 7,516 171 

WWTFs** 17,335 4,492

Ground Water Unquantified; assumed negligible 

by Scholl et al. 1980

Unquantified; assumed negligible 

by Scholl et al. 1980

Total Loading: 221,135

(13.04 g/m2/yr) 

37,590

(2.22 g/m2/yr) 

Nonpoint surface water drainage conveys the largest percentage of the total external nutrient loading to the 

HRE (78% to 89%), followed by WWTFs (8% to 21%). Atmospheric loading estimates, based on areal loading 

estimates for the north IRL (Gao 2009), constitute about 3% of the total TN loading and less than 1% of TP 

loading. 

Ground water nutrient loading to the HRE was considered negligible by Scholl et al. (1980), possibly because 

the majority of the population in the HRE catchment is served by centralized WWTFs, not septic tanks, and 

there are large withdrawals from both the surficial and Floridan aquifers for water supply (Rutledge 1985). 

Despite the conclusion reached by Scholl et al. (1980), the Floridan aquifer may be a significant, albeit 

unquantified, source of TP to the HRE. A geologic confining layer, the Hawthorn Formation, ordinarily restricts 

water movement between the Floridan and surficial aquifers. That layer is nearly absent in the HRE catchment 

(Toth 1987), and what remains is a thin, discontinuous clay strata with slags of apatite (ferric4

Estimating annual TP loading from the upper Floridan is difficult at best. The interannual variability of the 

Floridan potentiometric surface elevation in coastal Volusia County is so large, approximately between +10 

feet to -10 feet from mean sea level per year over the last 10 years (D. Toth, SJRWMD, pers. com., May 2010), 

that any estimate of upwelled TP loading in 1 year can easily be offset by zero loading in another year. 

Nonetheless, over the long term, the contribution from the Floridan via upwelling or pumped withdrawal for 

surface irrigation may be partly responsible for maintaining high TP in HRE waters.

phosphate) 

(J. Martin, University of Florida Geology Department, April 2010). Consequently, some degree of hydrologic 

connection exists between the aquifers, potentially allowing the artesian upwelling of phosphorus-rich ground

water into the HRE (~0.1 mg/L TP near the HRE vs. 0.03 mg/L TP near Mosquito Lagoon; SJRWMD upper 

Floridan well data). 

The total external TN and TP loading estimates for the north and south HRE were normalized to estuarine 

surface water area (g/m2

/yr) for input to the Steward and Lowe (2010) model. The north and south HRE areal 

loading estimates were then paired with their respective hydraulic loadings (mean depth/residence time) and 

plotted on a graph that describes TN or TP areal loading limit (published TMDLs or pollutant load reduction 

goals [PLRGs]) as a function of estuarine hydraulic loading (Figure 13). The line of best fit in the graph 

represents upper mesotrophy (UML) for Florida estuaries (based on Steward and Lowe 2010). The line in the 

plot representing lower mesotrophy (LML) was constructed using TN and TP limits for lower mesotrophy 

established by Steward and Lowe (2010), Smith (1998), and Baker et al. (1981)(further explanation provided in 

Steward and Lowe 2010). 

 

4 FDEP determined that iron (Fe) in the HRE exceeds the standard and commented that ground water may be the major source (FDEP 2008).
Draft 30 August 2010 

The plotted points for TN and TP are located on or just below the lower mesotrophic line (LML) for the south

HRE, indicating lower mesotrophy or oligo-mesotrophy; and just above the upper mesotrophic line (UML) for 

the north HRE, indicating eutrophy (Figure 13). Alternatively, the TSI values and chlorophyll a in the north HRE 

indicate (upper) mesotrophy. The inconsistency between these sets of results for the north HRE may be 

explained by high levels of tripton in the north HRE that attenuate the trophic response, as indicated by the 

low TSI, but this phenomenon cannot accounted for in the Steward and Lowe (2010) model. The same 

phenomenon may exist in the south HRE, but its current nutrient loading rates are sufficiently low to maintain 

oligo-mesotrophic conditions.

 

 

Figure 13. The log-linear plot of TN and TP load limits (g/m2

/yr) vs. hydraulic loading (qs). A point lying 

above the 95% confidence interval for the line-of-best-fit (= upper mestrophy) is considered eutrophic; below 

the lower mesotrophic line (LML) is considered oligotrophic (based on Steward and Lowe 2010).

The plotted positions of the TN and TP points on the Florida trophic models (Fig. 13) suggest that the current 

external nutrient loadings (ca 2004) to S. HRE can serve as the loading limits that should maintain that 

segment’s current trophic state (oligo-mesotrophy). The positions of the N. HRE points suggest that current 

N. HRE

S. HRE

-0.5

0.5

1

1.5

2

2.5

-0.5 0 0.5 1 1.5 2 2.5 3

log qs (z/τw; m/yr)

log TN load limit (g/m2/yr)

95% upper CI

LML

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

-0.5 0.0 0.5 1.0 2.0 2.5 3.0

log qs (z/τw; m/yr)

log TP load limit (g/m2/yr)

S. HRE

N. HRE

2.0

95% upper CI

LML

TN

TP

UML

UML

log TN Load Limit (≈UML) = 0.83(log qs) – 0.018; R2 = 0.96, p < 0.001

log TP Load Limit (≈UML) = 0.92(log qs) – 0.95; R2

TP log TP Load Limit (≈UML) = 0.92(log qs) – 0.95; R = 0.97, p < 0.001 2 = 0.97, p < 0.001
Draft 31 August 2010 

nutrient loadings may promote eutrophy in that segment; however, there is other evidence (mean TSI < 50; 

and the lack of impairment symptoms as described in Table 2) that indicate mesotrophy. It is likely that other 

factors (possibly including tripton) may be attenuating the trophic response in N. HRE. Without definitive 

evidence of eutrophy or eutrophic symptoms, the current nutrient loadings to N. HRE are regarded as the 

loading limits sufficient to maintain mesotrophy in that segment. 

Finally, the Dettmann (2001) and Steward and Lowe (2010) models were used to predict TN and TP

concentrations in the north and south HRE that correspond to their respective current loadings or loading 

limits. The TN and TP loading limits (current loadings) and their predicted concentrations are presented as the 

third line of evidence (Tables 10a and 10b, respectively). 

Table 10a. North HRE current loading (loading limits) for TN and TP with the model-predicted 

concentrations. * The Dettmann (2001) TN model equation was applied (Steward et al. 2010). 

**The upper mesotrophic [TP] model equation was applied to the north HRE (Steward et al. 2010). 

North HRE TN

(kg/yr)

TN

(mg/L)*

TP

(kg/yr)

TP

(mg/L)**

Current loading 

(circa 2004)

257,832 0.62 43,494 0.20

Table 10b. South HRE current loading (loading limits) for TN and TP with the model-predicted 

concentrations. * The Dettmann (2001) TN model equation was applied (Steward et al. 2010). 

** Midpoint between the two [TP] values reported in the parentheses. Two [TP] values are reported because both the lower mesotrophic and 

oligotrophic [TP] model equations were applied to the south HRE (Steward et al. 2010). 

South HRE TN

(kg/yr)

TN

(mg/L)*

TP

(kg/yr)

TP

(mg/L)**

Current loading 

(circa 2004)

221,135 0.42 37,590 0.135 (0.09, 0.18)

Proposed Numeric Nutrient Targets 

Methodology 

For consistent application and for providing an appropriate level of protection, water quality criteria need to 

include magnitude, frequency, and duration components. The magnitude is a measure of how much of a 

pollutant may be present in the water without an unacceptable adverse effect. Duration is a measure of how 

long the pollutant may be above the magnitude, and frequency relates to how often the magnitude may be 

exceeded without adverse effects. It is preferred to derive the magnitude component of a criterion through a 

cause-effect relationship (such as those measured through toxicity testing). The magnitude would then be set 

at a level that would protect a majority of the sensitive aquatic organisms inhabiting the system. Absent a 

demonstrated cause-effect relationship, the magnitude may be set at a level designed to maintain the current 

data distribution, accounting for natural temporal variability, when the current conditions are protective of the 

designated uses of the waterbody. Since a criterion derived based on the existing data distribution has no 

direct link to any observed cause and effect relationship, it is assumed that maintaining the current data 

distribution will preserve the uses associated with that distribution. 

The frequency and duration components of the criteria are best established as additional descriptors of the 

reference condition data distribution. Specifically, these components should be part of a statistical test 

designed to determine whether the long-term distribution of data has shifted upward (or in some cases, 

downward) from the reference distribution. This test would then be used to determine whether future 
Draft 32 August 2010 

monitoring data are consistent with the magnitude (long-term average) defined by the reference dataset. It is 

critical to account for the natural variability surrounding the magnitude expression and to control for statistical 

errors.

The derivation of the magnitude, frequency and duration components of the numeric nutrient criteria for 

Pensacola Bay is described briefly below. More details concerning the statistical approaches used can be 

found in the document “Overview of FDEP Approaches for Nutrient Criteria Development in Marine Waters” 

(FDEP 2010).

Magnitude 

The magnitude component can be set at a variety of values based on the frequency and duration that will be 

used to determine compliance. For the “healthy existing conditions” approach, FDEP established the 

magnitude at: a) an adjusted long-term central tendency (geometric mean) of the distribution, expressed as a 

long-term geometric mean never to be exceeded; and b) an annual geometric mean, not to be exceeded more 

than two times over a five- year period. The long-term average condition was adjusted to a level that 

represents an insignificant increase from the baseline. Allowing for an insignificant increase accounts for 

uncertainty in the estimate of the mean and partially addresses the fact that the reference based approach 

fails to identify a threshold of imbalance, and thus is highly prone to excessive Type I error. For the purpose of 

antidegradation evaluations, EPA has established that a ten percent change in a water quality parameter 

represents an insignificant departure from the historical condition, known as a de minimus change (US Sixth 

Circuit Court 2008). Therefore, the proposed thresholds allow for up to a ten percent increase in the long-term 

geometric mean, which was calculated as the geometric mean of the annual geometric means of the long-term 

dataset for each segment of the waterbody.

Because it is not always practical to assess compliance with the criteria on the same long-term basis, FDEP also 

developed criteria expressed as an annual geometric mean so that estuaries could be assessed on an annual 

basis. It should be noted that the duration and frequency components of the criteria must be linked to the 

response timeframe of the sensitive endpoint. Short-term averaging periods (e.g., 1 to 30 days) might be 

appropriate for nutrient criteria where a sufficiently robust cause-effect relationship has demonstrated that a 

eutrophic response occurs over such timeframes. If however, such a short-term response cannot be 

demonstrated, or there is no indication of use impairment, longer averaging periods should be used. 

Since the duration and frequency components of the criteria must be consistent with the derivation of the 

magnitude component, the long-term geometric mean target cannot simply be applied as an annual mean. 

Doing so would result in an unacceptability high Type I failure rate (identifying a healthy system as being 

impaired) since approximately 50 percent of the individual yearly means are expected to be above the longterm mean. Therefore, the long-term target must be adjusted to allow for the application to a shorter 

duration with an acceptable Type I error rate of no more than a 10 percent.

An annual criterion with an approximate 10 percent Type I error rate for a given frequency was derived by 

appropriately accounting for the annual variability above the mean. This annual target concentration was 

derived as an upper percentile of the distribution of the annual geometric mean concentrations. Previous 

proposals by EPA have utilized three-year assessment periods to express the duration nutrient criteria 

components. Although it is possible to construct a test that achieves the 10 percent Type I error rate target 

over a 3-year period, a slightly longer period (5 years) will provide better control for Type II error and will more 

fully capture climatic cycles (e.g., el nino, la nina), which tend to be longer than three years in Florida (and may 

actually be on a multi-decadal scale). Furthermore, a five year period is more consistent with both the State’s 

five year 303(d) assessment and NPDES permit renewal cycles.
Draft 33 August 2010 

Given a five year assessment period, FDEP derived criteria expressed as an annual geometric mean by using 

the 75th percentile of the annual geometric means from the long-term dataset. FDEP demonstrated that 

setting the criteria at this percentile results in a magnitude that, when expressed as an annual geometric mean 

not to be exceeded more than twice during a five-year period, will achieve the targeted 10 percent error rate. 

The magnitude component of the criteria (not to be exceeded more than twice in a 5 year period) was 

established to allow for two separate assessments:

As an annual geometric mean for a network of stations in a specified segment of the waterbody; and 

As an annual geometric mean for a single station in a given segment. 

The two expressions of magnitude differ in the handling of within-segment spatial variability. The network 

average magnitude was calculated based on the distribution of spatially averaged annual geometric means, 

while the single site magnitude was calculated using the distribution of station annual geometric means. Use 

of the network average magnitude is appropriate for assessing overall segment status. However, a network 

average is only representative of overall segment status if it is relatively homogeneous. If the segment is not 

well mixed, is very large, or contains localized nutrient sources, then there is a potential that isolated areas of 

enrichment may be masked in the network spatial average. Therefore, the single station annual geometric 

mean has the benefit of assessing localized enrichment patterns. DEP has not decided whether both 

expressions of magnitude are needed in all cases, and we are seeking additional feedback from local experts 

and stakeholders.

Summary and Conclusions

The results from the three lines of evidence are tabulated for ease of comparison (Tables 11a and 11b). 

Overall, the results from the HRE-specific regression models and the general models compare well with the 

reference period results, increasing the level of confidence that numeric nutrient criteria for the north and 

south HRE can be based on the 2000–08 period of record. For the south HRE, the results of the TN regression 

model do not compare well with those of the reference period method. This inconsistency cannot be 

explained, but it is the exception in an otherwise solid, weight-of-evidence case for the HRE. 

For the north HRE, the potential for current (circa 2004) nutrient loadings to induce eutrophy has possibly 

been mitigated by the estuary’s turbidity (more specifically, tripton or non-algal turbidity), suggesting that 

current nutrient loadings can serve as the loading limits protective of the system’s current trophic state 

(mesotrophy to oligo-mesotrophy). In addition, the three lines of evidence for the north HRE produced a tight 

range of annual and wet-season median concentrations (Tables 11a). Therefore, the north HRE current loading 

estimates (ca 2004) and reference period results are proposed as that segment’s nutrient criteria: 257,832

kg/yr TN and 43,494 kg/yr TP; 0.61 mg/L TN and 0.12 mg/L TP as annual medians; and 0.90 mg/L TN and 0.18 

mg/L TP as July–September medians (Table 11a). 
Draft 34 August 2010 

Table 11a. Summary of lines-of-evidence results: Loading and concentration limits for the north HRE. 

Target condition is mesotrophy (chlorophyll a targets are an annual median of 4.5 μg/L and a seasonal median of 7.9 

μg/L for July through September) 

- = Empty cell/no data

kg/yr = Kilograms per year

mg/L = Milligrams per liter

N/A = Not applicable a 0.62 mg/L TN and 0.2 mg/L TP are predicted concentrations based on current loadings (ca. 2004). 

- TN 

(kg/yr) 

TN 

(kg/yr) 

TP 

(kg/yr)

TP 

(kg/yr) 

Loading Limit (current loading) 257,832 257,832 43,494 43,494

- Median (mg/L)

Annual

Median (mg/L)

July–September

Median (mg/L)

Annual

Median (mg/L)

July–September

Reference Period (2000–08) 0.61 0.90 0.12 0.18

Regressions (1992–2008 data) 0.58 0.90 Not significant 0.17

General Models 0.62, a N/A 0.2, a N/A

Table 11b. Summary of lines-of-evidence results: Loading and concentration limits for the south HRE. 

Target condition is oligo-mesotrophy (chlorophyll a targets are an annual median of 3.5 μg/L and a seasonal median of 

5.8 μg/L for July through September)

- = Empty cell/no data

N/A = Not applicable 

a The midpoint between the two values in parentheses = 0.135 mg/L TP. The two [TP] values reported in parentheses are the results from the lower 

mesotrophic and oligotrophic [TP] model equations applied to the south HRE (see the section on General Considerations and Methods). 

- TN 

(kg/yr)

TN 

(kg/yr)

TP 

(kg/yr)

TP 

(kg/yr)

Loading Limit (current loading) 222,000 222,000 38,000 38,000

- Median (mg/L) 

Annual

Median (mg/L)

July–September

Median (mg/L)

Annual

Median (mg/L)

July–September 

Reference Period (2000–08) 0.39 0.51 0.12 0.19

Regressions (1992–08 data) 0.30 0.41 Not significant 0.15

General Models 0.42 N/A 0.135 (0.09, 0.18)a N/A

For the south HRE, the current (circa 2004) loadings of TN and TP are proposed as loading limits. Current TN 

and TP loadings should maintain the oligo-mesotrophic condition in the south HRE. The predicted trophic 

state and concentration limits from the regression analysis and the general models support that conclusion 

and are generally consistent with the 2000–08 reference period results (Table 11b). Consequently, the south

HRE current loadings (ca 2004) and reference period results are proposed as that segment’s nutrient criteria: 

222,000 kg/yr TN and 38,000 kg/yr TP; 0.39 mg/L TN and 0.12 mg/L TP as annual medians; and 0.51 mg/L TN 

and 0.19 mg/L TP as the July through September medians (Table 11b). 
Draft 35 August 2010 

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